Hybrid optical devices have already been made in the prior art. In particular, elements exist that are as shown in the perspective diagram of FIG. 1A. Such an element includes a passive component 1 made of silica and an opto-electronic component 2, e.g. of the laser type or of the optical amplifier type, both components being supported by a substrate 3 made of silicon.
The passive component 1 is, in fact, made up of a stack of three successive layers of silica 6, 7, and 8 on the silicon substrate 3. The second layer 7 of silica is etched into a special shape, and it constitutes the light guide of the passive component 1. In the example shown in FIG. 1A, the light guide is rectilinear, but it may be in some other shape depending on the function that it is to perform. The second layer of silica 7 is so made that it has a higher refractive index than the other two layers 6 and 8. The second layer 7 is thus equivalent to the core of an optical fiber, while the other two layers 6 and 8 are equivalent to the cladding of such an optical fiber.
The assembly made up of the silicon substrate 3 and of the layers of silica 6, 7, and 8 forming the passive component is commonly referred to as a silica-on-silicon structure 10 and abbreviated "SiO.sub.2 /Si"
A recess 5 is defined in the layers of silica 6, 7, and 8, and in the vicinity of the silicon substrate 3. The opto-electronic component 2 is then placed inside the recess 5 such that its optical axis 9 is in alignment with the light guide 7 of the passive component 1.
The alignment between the optical axis 9 of the active component 2 and the light guide 7 of the passive component 1 must be very accurate, i.e. accurate to within about one micron (.mu.m) or less (1 .mu.m=10.sup.-6 meters). For this purpose, the cavity 5 must have a structure that enables the opto-electronic component 2 to be locked in position, as shown in the cross-section view of FIG. 1B. As shown in FIG. 1B, the cavity 5 is provided with steps 51 so as to match the shape of the active component. In addition, the dimensions of the cavity 5 are defined with accuracy to within one micron or less. A solder spot S enables the active component 2 to be fixed to the bottom of the cavity.
The arrows shown in FIG. 1B indicate the direction in which the heat given off by the opto-electronic component 2 is removed.
That type of hybrid optical element is disclosed, in particular, in the document entitled "Thermal resistance and soldering stress analyses on LD-mount-structures for planar lightwave circuits", I. Kitazawa et al. IOOC '95, FA3-5, pp 26-29.
However, that type of hybrid optical element is only in very limited use. This is essentially because silicon has a coefficient of thermal expansion that is very different from the coefficient of thermal expansion of silica. As a result, after the successive layers 6, 7, and 8 of silica have been deposited at a temperature lying in the range 400.degree. C. to 1400.degree. C. on the silicon substrate, and after the resulting assembly has been cooled to ambient temperature, the silica-on-silicon structure thus obtained has a curved, very deformed shape. That deformation gives rise to stresses that propagate through the various layers of silica.
Such stresses are not problematic when a straight guide is to be made, but they do constitute a major drawback when other types of passive component are to be made such as, for example, a power divider, i.e. a coupler having 3 dB losses.
Optical polarization rotates in optical fibers. When light coming from one optical fiber is injected into a 3 dB coupler, it is essential to have a component that is capable of dividing the optical power regardless of polarization. The stresses generated in the layers of silica make the passive function of such a coupler sensitive to polarization, and, as a result, that type of passive component cannot operate correctly.
One solution to that problem is shown in FIG. 1C and consists in using a support 3 made of fused silica, i.e. of amorphous silica. Such a structure is referred to as a silica-on-silica structure and is abbreviated "SiO.sub.2 /SiO.sub.2 ". In the example shown, the passive component is a coupler C. In that case, the substrate 3 and the layers of silica defining the coupler C have coefficients of thermal expansion that are very close to each other so that no stress is generated in the layers of silica. The passive component is then insensitive to polarization.
Such a solution is described, for example, in the article entitled "Prospects for silica and glass-based IO components" by S. Kobayashi et al, ECIO '95, We A3, pp 309-314.
However, unlike silicon, fused silica is a poor dissipator of heat. Unfortunately, when making a hybrid optical element, it is necessary to remove the heat given off by the opto-electronic component.
The problem to be solved thus consists in finding means making it possible to make a hybrid optical element on a fused silica substrate, such an element including both an active component and a passive component, the two components being coupled optically to each other.
A first solution that comes to mind to solve that problem is shown in the diagrammatic cross-section view given in FIG. 2A. That solution consists in covering the active component 2 with a silicon support 4 suitable for dissipating the heat given off by the active component. The direction in which the heat given off is removed is indicated by arrows in FIG. 2A.
However, that first solution is not optimal because it suffers from a major drawback. The heat is removed via the active component 2 and that upsets operation of the component considerably. In addition, the qualities of the opto-electronic component are reduced considerably, such as is its life span.
Moreover, in that case, the means for holding the silicon support 4 are not defined. Said the support can optionally be glued to the silica-on-silica structure. In any event, the support does not contribute to aligning the active component with the passive component. It is only the shape of the cavity 5 and the solder spot S that make it possible to achieve the alignment, as in the above-described prior art corresponding to FIGS. 1A and 1B.
To avoid heat being removed via the opto-electronic component, another solution consists in turning the component the other way up as shown in FIG. 2B. In that case, the heat is removed to the silicon support 4 via the shortest route, without passing through the active component 2. The silicon support 4 acts as a heat pathway.
The back face 2a of the opto-electronic component 2 is thinned by polishing with an accuracy of .+-.10 .mu.m, without reference to the active function 9. The heat pathway 4 is mounted in the vicinity of the optical axis 9 of the opto-electronic component 2. Such a heat pathway is intended to act as a heatsink only. In addition, it can optionally serve to encapsulate the chip 2. But it can in no way contribute to aligning the optical axis 9 of the active component correctly relative to the light guide of the passive component 1 because such an alignment function is incompatible with the large tolerances (.+-.10 .mu.m) for thinning of the back face 2a of the opto-electronic component. As a result, the second solution is also unsatisfactory.
Another type of hybrid optical device has been proposed in the prior art, and it is shown in the diagrammatic longitudinal section view given in FIG. 2C. That device includes an opto-electronic component 2, a heat pathway or "bridge" made of silicon 4, and a silica-on-silicon structure 10 on which at least one passive component is implemented. That structure 10 can also be a silica-on-silica structure.
In that case, the silicon bridge 4 serves both as a heatsink and as a support that is common to the active component 2 and to the silica-on-silicon structure 10.
To support the two types of component, it is thus necessary for the area of the bridge 4 to be relatively large, i.e. typically greater than 0.1 cm.sup.2. That area can be as much as a few square centimeters. The silica-on-silicon structure 10 is fixed to the bridge 4 at two points referred to as "support points" A and B. The distance between the two support points A and B is of the same order of magnitude as the size of the structure 10. It is therefore relatively long, and can lie in the range 1 mm to a few centimeters.
That long distance gives rise to problems of shear stress generated at the support points A and B during standard telecommunications applications which are performed at temperatures that can vary in the range -40.degree. C. to +85.degree. C. Such shear stress problems are due to the difference between the coefficients of thermal expansion of the silicon of the bridge 4 and of the silica of the silica-on-silicon structure 10 or of the silica-on-silica structure.